# Tension field time pronounced zigzag pattern

**URL:** <https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237>\
**Category:** Uncategorized\
**Created:** [August 27, 2022, 4:22pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237 "2022-08-27T16:22:15Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Nobody-86](https://avatars.discourse-cdn.com/v4/letter/n/e495f1/32.png) [@Nobody-86](https://calculix.discourse.group/u/Nobody-86)\
**Post date:** [August 27, 2022, 4:22pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237/1 "2022-08-27T16:22:15Z")

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Hello there,

I have worked out a plastic calculation, and the stress field of the first principal stress shows a distinct zig-zag pattern. The model consists of Quadratic Hexahedral Elements (C3D20). The pattern occurs because of discontinuous results on corner and midpoint nodes.

How can this happen and what can I do about it? Linear elements produce a continuous pattern.

I am using version 2.20 on Windows 10.

 ![stress](https://global.discourse-cdn.com/free1/uploads/calculix/original/1X/61db3ab60c5624372b926ae9bbc0994a8e42de3b.png)

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**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\
**Post date:** [August 27, 2022, 4:47pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237/2 "2022-08-27T16:47:22Z")

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Can you share the input file ? If not, please show the whole model and describe how it’s loaded and constrained. Have you tried with other element types ? The choice of element type in CalculiX is particularly important when plasticity is involved: [Elasto-plastic bending - elements, accuracy](https://calculix.discourse.group/t/elasto-plastic-bending-elements-accuracy/700)

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<div class="post-metadata">

**Author:** ![Nobody-86](https://avatars.discourse-cdn.com/v4/letter/n/e495f1/32.png) [@Nobody-86](https://calculix.discourse.group/u/Nobody-86)\
**Post date:** [August 27, 2022, 5:22pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237/3 "2022-08-27T17:22:07Z")

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The model is a simple notched tensile specimen. By using the symmetry I have modeled only 1/8 and given the symmetry surfaces with corresponding boundary conditions (x=0, y=0, z=0). At the upper surface I give up a displacement (y=10)

Here is my input file:

```auto
*INCLUDE, INPUT=all.msh
*INCLUDE, INPUT=sym_x.nam
*INCLUDE, INPUT=sym_y.nam
*INCLUDE, INPUT=sym_z.nam
*INCLUDE, INPUT=force.nam

*MATERIAL, Name=steel
*ELASTIC
200e3, .3
*PLASTIC, HARDENING=ISOTROPIC
400.000000, 0.000000
500.000000, 0.042179
600.000000, 0.170387
700.000000, 0.496688
800.000000, 1.225562
900.000000, 2.700146
1000.000000, 5.460853

*SOLID SECTION, Elset=Eall, Material=steel

*STEP, NLGEOM
*STATIC
    .001, 1

*BOUNDARY
    Nsym_x, 1, 1, .0
    Nsym_y, 2, 2, .0
    Nsym_z, 3, 3, .0
    Nforce, 2, 2, 10.0

*NODE FILE
U
*EL FILE
S, E, PEEQ
*NODE PRINT, NSET=Nsym_y
RF

*END STEP

```

Here is a picture of the whole model. Nsym\_y is “under” the model, and Nsym\_z is “behind” the model.

 ![model](https://global.discourse-cdn.com/free1/uploads/calculix/original/1X/317799d09be49236d42bb725f275705ece6e1bc7.png)

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<div class="post-metadata">

**Author:** ![xyont](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/xyont/32/3553_2.png) [@xyont](https://calculix.discourse.group/u/xyont)\
**Post date:** [August 27, 2022, 6:56pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237/4 "2022-08-27T18:56:28Z")

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> [@Nobody-86](#):
>
> The model consists of Quadratic Hexahedral Elements (C3D20)

you can try using reduced integration type (C3D20R) to see the differences since full integration is not preferred to be use in plastic analysis.

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<div class="post-metadata">

**Author:** ![Nobody-86](https://avatars.discourse-cdn.com/v4/letter/n/e495f1/32.png) [@Nobody-86](https://calculix.discourse.group/u/Nobody-86)\
**Post date:** [August 28, 2022, 12:52pm UTC](https://calculix.discourse.group/t/tension-field-time-pronounced-zigzag-pattern/1237/5 "2022-08-28T12:52:47Z")

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Thank you very much. I changed the elements to C3D20R and now the result looks reasonable.

 ![stress2](https://global.discourse-cdn.com/free1/uploads/calculix/original/1X/1cf6f5db92f087d7c87c4e69fbfab58caaaf0e71.png)